1. Field of Invention
The present invention relates generally to a method for producing a composite material comprising a matrix phase and a dispersed phase, in particular a metal-metal or a metal-ceramic composite material, such as a tungsten-copper composite material, and the material produced thereby.
2. Description of Related Art
Metal-metal composite and metal-ceramic composite materials are popular as special materials in plant apparatus and equipment construction due to their enhanced mechanical, electrical and thermal properties. In electrical and electronic applications, tungsten-copper composites are often employed owing to their high wear resistance and superior thermal and electrical properties.
For the production of composite materials, in particular tungsten-copper composites, various processes are known. These processes, however, have their limitations in the aspects of quality of composites produced, process speed and economic considerations.
Composite materials consist of two phases—a matrix phase which is continuous and surrounds the other phase often known as a dispersed phase. For example, in the context of tungsten-copper composites, tungsten forms the matrix phase and copper forms the dispersed phase within the tungsten matrix. The quality of the composite material is determined by its homogeneity and porosity, i.e., even distribution of copper throughout the tungsten-copper composite and low percentage of voids formed.
Composites are multiphase materials that exhibit a significant proportion of the properties of both constituent phases such that a better combination of properties is realized. Therefore, a uniform distribution of the two phases throughout the composite is required to ensure homogeneous material properties.
Porosity is deleterious to flexural strength, electrical and thermal conductivity of the composite. The presence of pores in the composite structure reduces the cross-sectional area across which a load is applied and they also act as points of stress concentrations, thus resulting in an exponential decrease in flexural strength. Air that is present in the pores has poor thermal and electrical conductivity, and thus affects the overall thermal and electrical properties of the composite. Therefore, it is desirable to minimize formation of pores in the composite during its manufacturing process.
Present manufacturing technologies available for producing metal or metal-ceramic composite materials, in particular tungsten-copper composites, include powder metallurgy compacting, covering and infiltration (also known as sinter casting), powder metallurgy compacting, covering and infiltration under pressure (also known as pressure casting), powder injection molding, covering and infiltration, and powder injection molding of a composite feedstock.
In powder metallurgy compacting, covering and infiltration, a first metal matrix or ceramic/carbide matrix, having a higher melting point and having a network of interconnected pores, is produced by powder metallurgy compacting, which fabricates the matrix by compacting a metal or ceramic/carbide powder into a mold under high pressure and then sintering the compacted powder to form the matrix. Solid plates of a second metal, having a lower melting point, are placed on the surface of the matrix to cover it, and are melted under a high temperature to enable infiltration of the second metal by capillary action into the matrix to fill up the pores. The metal filled pores form the dispersed phase. However, the matrix produced by powder metallurgy compacting has an uneven distribution of pores which results in a non-uniform distribution of the dispersed phase in the composite.
In powder metallurgy compacting, covering and infiltration under pressure (also known as pressure casting), a first metal matrix or first ceramic/carbide matrix, having a higher melting point and having a network of interconnected pores, is produced by powder metallurgy compacting. A second metal, having a lower melting point and in a liquid state, is placed in a mold with the matrix. This is followed by infiltration of the second metal into the pores of the matrix by means of an external applied pressure. Yet again, the matrix resulting from powder metallurgy compacting has an uneven distribution of pores which results in a non-uniform distribution of the dispersed phase in the composite. Further, the use of an applied pressure substantially increases manufacturing costs.
In powder injection molding, covering and infiltration, a first metal matrix or first ceramic/carbide matrix, having a higher melting point and having a network of interconnected pores, is produced by powder injection molding (PIM). In PIM, the matrix is fabricated by injecting a PIM feedstock, the PIM feedstock comprising a metal or ceramic/carbide powder and binder, into a mold where it is cooled and then ejected therefrom. The binder is removed from the ejected material, which is then sintered to form the matrix. Solid plates of a second metal, having a lower melting point, are placed on the surface of the matrix. Infiltration of the second metal into the matrix is completed by capillary force action under high temperatures. This method has an advantage in that it results in a composite material with a more even distribution of the dispersed phase within the matrix. However, this method is only suitable for producing composites that are of a simple geometry and is not suitable for producing composite components with complicated shapes. Further, the method involves separately providing a metal plate on the surface of the matrix for infiltration to take place.
In powder injection molding of composite powders, the composite powder is a mixture of metal/ceramic/carbide powder with binders, which is known as the PIM feedstock. This process fabricates the composite component by first injecting a heated PIM feedstock into a mold where it is cooled and from which it is then ejected. This is followed by removing the binder from the ejected material, and then sintering the material to form the composite component. This method, although achieved in a single process, is limited in its inability to produce composite components that have a high composition of the dispersed phase. For example, in the context of tungsten-copper composites, composites with 20–30 weight % of copper are very difficult to produce by this method, owing to the large density difference between tungsten and copper, as well as the lack of tungsten to tungsten particle interlocking. This causes copper to bleed out during sintering which leads to loss of copper and defects in the composite component such as formation of voids in its microstructure.
U.S. Pat. No. 5,963,773, issued on 5 Oct. 1999, to Yoo, et al., discloses a method of fabrication of a tungsten skeleton structure comprising the steps of forming a source powder by coating a tungsten powder surface with nickel, forming an admixture by admixing the source powder and a polymer binder, performing powder injection molding and obtaining a tungsten skeleton structure by removing the polymer binder. A copper plate is then placed beneath the tungsten skeleton structure and copper infiltration is carried out at a temperature between 1150° C. and 1250° C. within a hydrogen atmosphere for 2 hours. However, the method involves separately providing a copper plate beneath the tungsten skeleton structure for copper infiltration to take place. Further, this method is not viable or too troublesome for producing components with complicated shapes.
U.S. Pat. No. 5,574,959, issued on 12 Nov. 1996, to Tsujioka, et al., relates to a process for manufacturing composites comprising the steps of mixing tungsten powder and nickel powder to form a mixed metal powder, kneading the mixed metal powder with an organic binder to form an admixture, injection molding the admixture to form a pre-determined shape, removing the binder from the shaped material, and applying a surface powder to at least one surface of the shaped material to prevent effusion of copper during sintering. The shaped material is then placed on a plate of solid copper and placed in a sintering oven where the copper melts and infiltrates into the shaped material. However, this method also involves separately providing a copper plate beneath the shaped material for copper infiltration to take place and is not viable or too time consuming for producing components with complicated shapes.
U.S. Pat. No. 5,413,751, issued on 9 May 1995, to Polese, et al., describes a process for forming heats sinks and other heat dissipating elements by press-forming composite powders of metal components, for example tungsten and copper, to form pressed compacts and then sintering the pressed compacts to achieve a homogeneous distribution of the copper throughout the tungsten-copper composite structure. However, the use of an external pressure to compact the composite powders leads to substantial increase in manufacturing costs.
At least some of the above processes might usefully be improved upon.